Tim Peters added the comment:
This is how it's supposed to work: Python's re matches at the leftmost
position possible, and _then_ matches the longest possible substring at that
position. When a regexp _can_ match 0 characters, it will match starting at
index 0. So, e.g.,
>>> re.search('(a*)', 'caaaat').span()
(0, 0)
shows that the regexp matches the empty slice 'caaaat'[0:0] (the leftmost
position at which it _can_ match), and
>>> re.search('(a(b+)a){0,1}', 'caabbaat').span()
(0, 0)
shows the same. The groups didn't match anything in this case, because the
outer {0,1} said "it's OK if you can't match anything". Put another group
around it:
>>> re.search('((a(b+)a){0,1})', 'caabbaat').groups()
('', None, None)
to see that the regexp as a whole did match the empty string.
----------
nosy: +tim_one
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<http://bugs.python.org/issue17257>
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